There’s a pungent cloud of hype and hope around probiotics—and researchers have long tried to clear the air about what the bowel-blasting products can ( and mostly can’t) do. Now, a new set of studies offers a gut-check on funky claims, ripping current probiotics as likely ineffective at boosting health and potentially even causing harm.
In the two studies, both published this week in the journal Cell, Israeli researchers report that bacteria taken in supplements, aka probiotics, often have little impact on healthy people’s innards and, at worst, can elbow out native populations of microbes.
In the first study, the researchers found that healthy microbial populations in people’s plumbing tends to flush out the newcomers. Thus, the microbial interlopers from supplements have little impact on resident microbiomes—and, by extension, consumers’ health—and are largely just pooped out.
But probiotic strains can more easily take root in the gut if a person takes a strong dose of antibiotics that beats back their beneficial bacteria, the researchers found in the second study. This finding might suggest that the living supplements could help rejuvenate the intestinal inhabitants after an antibiotic onslaught—as probiotic makers would surely like to claim. But in fact, probiotics made it harder for the healthy, native community of gut bugs to recover, the researchers found. People who took probiotic supplements to rally their microbiome after antibiotics didn’t regain their healthy communities for as long as five months afterward. People who didn’t take anything after their antibiotics did.
The stinky findings are not shocking given past disappointing and inconclusive data on probiotics. A 2016 review of randomized controlled studies concluded that probiotics had almost no effect on the overall mix of microbes in people’s poop. While other studies have pushed out evidence of microbial changes from probiotics, there’s still little data on what those changes might mean in terms of health and what our microbiomes are actually doing for us. And previous studies looking specifically at probiotic use after antibiotics also found the supplements were ineffective. For instance, probiotics couldn’t stop up antibiotic-induced diarrhea or thwart the more serious antibiotic-associated gut invader, Clostridium difficile.